According to the state of the art, automatic transmissions, especially for motor vehicles, comprise planetary gear sets, which are shifted by means of friction and/or shifting elements, such as clutches and brakes, and are typically connected to a starting element that is subject to a slip effect and optionally provided with a lock-up clutch, for example a hydrodynamic torque converter or a fluid clutch.
Such automatic transmissions are known from DE 199 49 507 A1 of the applicant, for example, wherein a multi-step transmission is disclosed in which two non-shiftable front-mounted gear sets are provided on the input shaft, which on the output side generate two speeds, which, apart from the speed of the input shaft, may optionally be shifted to a shiftable double planetary gear set acting on the output shaft by selectively engaging the used shifting elements such that, for changing from one gear to the next higher or lower gear, only one shifting element of the two just actuated shifting elements has to be engaged or disengaged. In this case, seven forward gears are achieved using five shifting elements, and nine or ten forward gears are achieved using six shifting elements.
Moreover, a multi-speed automatic transmission with eight forward gears and one reverse gear is known from DE 102 13 820 A1, comprising a first input path T1 of a first transmission ratio: an input path T2, which has a higher transmission ratio than this input path T1; a Ravigneaux-type planetary gear set with four elements, the four elements being a first element, a second element, a third element, and a fourth element in the sequence of the elements in a speed diagram; a clutch C-2,which transfers the rotation of the input path T2 to the first element S3; a clutch C-1, which transfers the rotation from the input path T2 to the fourth element S2; a clutch C4, which transfers the rotation from the input path T1 to the first element; a clutch C-3, which transfers the rotation from the input path T1 to the second element C3; a brake B-1, which engages the fourth element; a brake B-2, which engages the second element, and an output element, which is coupled to the third element S3.
A 9-speed multi-step transmission is further known from DE 29 36 969 A1; it comprises eight shifting elements and four gear sets, one gear set being used as a front-mounted auxiliary transmission, and the main transmission having a Simpson set and a further gear set used as a reversing drive.
Other multi-step transmissions are known, for example, from DE 102005010210 A1 and DE 102006006637 A1 of the applicant.
Automatically shiftable vehicle transmissions in planetary design have generally been described in the state of the art on many occasions, and are subject to continuous development and improvement. Accordingly, these transmissions are expected to require a low construction cost, in particular a small number of shifting elements, and to prevent double shifts during sequential gear changes, so that only one shifting element at a time is changed when shifting in defined gear groups.
From the unpublished DE 102008000428.3 of the applicant, a multi-step transmission in planetary design is known, which has an input element and an output element that are arranged in a housing. In the known transmission, at least four planetary gear sets are provided, hereinafter designated as first, second, third, and fourth planetary gear sets, at least eight rotatable shafts, hereinafter designated as input shaft, output shaft, third, fourth, sixth, seventh, and eighth shaft, as well as at least six shifting elements, comprising brakes and clutches, whose selective engagement generates different transmission ratios between the input element and output element, so that preferably nine forward gears and one reverse gear can be implemented.
In this case, the first and second planetary gear set, which are preferably configured as minus planetary gear sets, form a shiftable front-mounted gear set, the third and fourth planetary gear sets forming a main gear set.
With respect to the known multi-step transmission, it is provided that the carriers of the first and second planetary gear sets are coupled to one another via the fourth shaft, which is connected to an element of the main gear set, that the ring gear of the first planetary gear set is coupled to the sun gear of the second planetary gear set via the eighth shaft, which can be detachably connected via a first clutch to the input shaft, and that the sun gear of the first planetary gear set can be coupled by means of the third shaft to a transmission housing via a first brake, and can be detachably connected to the input shaft via a second clutch, while the ring gear of the second planetary gear set can be coupled by means of the fifth shaft to a transmission housing via a second brake. In addition, the seventh shaft is permanently connected to at least one element of the main gear set and can be coupled to a transmission housing via a third brake, while the sixth shaft is permanently connected to at least one further element of the main gear set and can be detachably connected to the input shaft via a third clutch; the output shaft is permanently connected to at least one further element of the main gear set.
In the known transmission, the fourth shaft is preferably permanently connected to the ring gear of the third planetary gear set, while the sixth shaft can be permanently connected to the ring gear of the fourth planetary gear set and to the carrier of the third planetary gear set, and can be detachably connected via the third clutch to the input shaft. Further, the seventh shaft is permanently connected to the sun gears of the third and fourth planetary gear set, and can be coupled via the third brake to a transmission housing. In this connection, output is via the output shaft that is permanently connected to the carrier of the fourth planetary gear set. Furthermore, the third and fourth planetary gear sets can be combined and/or reduced to a Ravigneaux set with a common carrier and a common ring gear.